rocher I checked your demo page - the sidebar-2 modules don't sit under the main content in responsive mode until the screen-size is around 765px (as per screenshot below)...
Please view screenshot wherein
2) Col-xs class selector applies when screen size goes below 768px and
1) min-height being injected is being superseded by !important class selector due to which inline min-height doesnt apply henceforth, no gap issues in certain responsive screen size as per demo site page
You may also view the guide emphasizing on varied bootstrap grid system/classes
So currently, everything working well with my website EXCEPT that there's a large gap between More Blog Posts and the Subscribe modules etc when the screen-size is between 965 and 744px (approximately)
Solutions being proposed :
1) You may either make changes as per Post#19 to get it right in line with demo site page layout
2) Try add this to custom.css file:
@media only screen and (min-width: 768px) and (max-width: 992px){.t3-sidebar-right,#t3-content{min-height: 0 !important;}}
or
@media only screen and (min-width: 768px) and (max-width: 992px){.t3-sidebar-right{min-height: 0 !important;}}
Kindly ensure to clear your browser cache if changes dont reflect